diff --git a/.github/workflows/fullsend.yaml b/.github/workflows/fullsend.yaml
index 6552b6f..d35d6ab 100644
--- a/.github/workflows/fullsend.yaml
+++ b/.github/workflows/fullsend.yaml
@@ -43,25 +43,25 @@ jobs:
       group: fullsend-dispatch-${{ github.event.issue.number || github.event.pull_request.number }}
       cancel-in-progress: false
     if: >-
-      github.event_name != 'issue_comment'
-      || github.event.comment.user.type != 'Bot'
+      (github.event_name != 'pull_request_target' && github.event_name != 'pull_request_review'
+       || github.event.pull_request.head.ref != 'fullsend/scaffold-install')
+      && (github.event_name != 'issue_comment'
+       || github.event.comment.user.type != 'Bot')
     uses: fullsend-ai/.fullsend/.github/workflows/dispatch.yml@main
     with:
       event_action: ${{ github.event.action }}
 
   stop-fix:
+    # Job-level if: is intentionally coarse — it only screens for the
+    # /fs-fix-stop command on a PR from a non-bot. The authoritative
+    # authorization decision (collaborator permission API + PR-author escape
+    # hatch) is made in the step below, so a maintainer whose author_association
+    # is not MEMBER (e.g. private org membership) is not filtered out (ADR 0054).
     if: >-
       github.event_name == 'issue_comment'
         && github.event.issue.pull_request
         && github.event.comment.user.type != 'Bot'
         && github.event.comment.body == '/fs-fix-stop'
-        && (
-          github.event.comment.author_association == 'OWNER'
-          || github.event.comment.author_association == 'MEMBER'
-          || github.event.comment.author_association == 'COLLABORATOR'
-          || github.event.comment.author_association == 'CONTRIBUTOR'
-          || github.event.comment.user.login == github.event.issue.user.login
-        )
     runs-on: ubuntu-24.04
     permissions:
       contents: read
@@ -73,7 +73,37 @@ jobs:
           GH_TOKEN: ${{ github.token }}
           PR_NUMBER: ${{ github.event.issue.number }}
           REPO: ${{ github.repository }}
+          COMMENT_USER_LOGIN: ${{ github.event.comment.user.login }}
+          ISSUE_USER_LOGIN: ${{ github.event.issue.user.login }}
         run: |
+          set -euo pipefail
+          # ADR 0054: authorize via the collaborator permission API
+          # (admin|maintain|write), not author_association — the latter grants
+          # contributor status to anyone with a single merged PR (issue #5421).
+          # Mirrors has_repo_permission() in dispatch.yml; keep the two in sync.
+          # The PR author may always stop the fix agent on their own PR.
+          authorized=false
+          if [[ -n "$COMMENT_USER_LOGIN" && "$COMMENT_USER_LOGIN" == "$ISSUE_USER_LOGIN" ]]; then
+            authorized=true
+          else
+            if api_err=$(mktemp); then
+              if role=$(gh api "repos/$REPO/collaborators/$COMMENT_USER_LOGIN/permission" \
+                --jq '.role_name' 2>"$api_err"); then
+                case "$role" in
+                  admin|maintain|write) authorized=true ;;
+                esac
+              else
+                echo "::warning::Permission API call failed for $COMMENT_USER_LOGIN: $(cat "$api_err")"
+              fi
+              rm -f "$api_err"
+            else
+              echo "::warning::Failed to create temp file for permission check of $COMMENT_USER_LOGIN"
+            fi
+          fi
+          if [[ "$authorized" != "true" ]]; then
+            echo "::notice::User $COMMENT_USER_LOGIN is not authorized to stop the fix agent (requires write access or PR authorship)"
+            exit 0
+          fi
           gh label create "fullsend-no-fix" --repo "$REPO" \
             --description "Skip bot-triggered fix agent runs" --color "FBCA04" \
             --force 2>/dev/null || true